A recent report by a team of physicists at
McGill University concluded that in order to build smaller yet
still high-performing computer chips, designers will need to focus
on better understanding how an electrical charge behaves when
confined to metal wires only a few atoms thick.

Upper interconnect layers on an Intel
80486 DX2 taken with an optical microscope at 200x
magnification.

A very telling report

In collaboration with General Motors R&D,
the McGill team published a new paper in Proceedings of the
National Academy of Sciences that describes their investigation of
an ultra-small contact between gold and tungsten, two metals that
are currently used in computers chips to connect the different
functional components of a device.

During experimentation, they used advanced
microscopy techniques to image a tungsten probe and gold surface
with atomic precision, bringing both metals together mechanically
in a precisely controlled manner. They discovered that when they
did this, the electrical current was much lower than expected.

The group found that crystal defects, that is,
displacements of the normally perfect arrangement of atoms,
generated by bringing the two materials into mechanical contact
with one another was a reason for the reduction of current.

State-of-the-art electrical modeling by the
research group confirmed this result. It also showed that
dissimilarities in the electronic structure of two metals
contributed to the fourfold decrease in current flow —
even in instances when a perfect interface was used.

“You could use the analogy of a water
hose,” he explains. “If you keep the water
pressure constant, less water comes out as you reduce the diameter
of the hose. But if you were to shrink the hose to the size of a
straw just two or three atoms in diameter, the outflow would no
longer decline at a rate proportional to the hose cross-sectional
area; it would vary in a quantized (‘jumpy’)
way.”

It’s an important discovery in this
rapidly developing field of technology, especially as the size of
features in electronic circuits continues to shrink year after
year. “The size of that drop is far greater than most
experts would expect — on the order of 10 times
greater,” notes Prof. Grütter.

Outlook

Such a sharp reduction in current presents a
major hurdle for designers of future computer chips as it will
affect their choice in material along with the design of devices in
the emerging field of nanoelectronics. Further research is now
needed to look into ways to get around this problem, whether
it’s to go with different materials or otherwise change
current processing techniques.

“The first step toward finding a
solution is being aware of the problem,” Grütter
concludes. “This is the first time that it has been
demonstrated that this is a major problem for nanoelectronic
systems.” ■